1. Field of the Invention
The present invention relates to a rack assist type electric power steering system, and in particular to an improved rack assist type electric power steering system which is capable of decreasing a processing section of a ball screw by improving an installation position and structure of a ball screw, protecting a connector and sensor of a motor assembly from a heat source and preventing any interference of parts due to a drooping of a rack bar.
2. Description of the Background Art
Generally, an electric power steering system is directed to increasing a steering force using an electric force. In particular, a rack assist type electric power steering system (R-EPS) is directed to reciprocating a rack bar using a motor assembly installed between a rack bar and a rack housing. FIG. 1 is a view illustrating a conventional rack assist type electric power steering system.
As shown therein, a torque sensor 20 is installed in a portion which is vertical from an intermediate portion of a rack housing 10, and an input shaft 21 is installed in the interior of the same for thereby receiving a rotational force when a driver rotates a steering wheel. A pinion gear 22 is installed in a lower portion of the input shaft 21.
In addition, a rack gear 12 engaged with the pinion gear 22 is installed in the interior of the rack housing in an intermediate portion of the same. A support yoke is installed in a rear side of the rack gear 12 for thereby constantly pushing the same in the direction of the pinion gear 22. A tie rod 30 is installed at both sides of the rack bar 11 and is connected with a tire wheel of a vehicle for thereby steering the same.
A motor assembly 40 is installed in the other side of an intermediate portion of the rack housing 10. The motor assembly 40 includes a motor housing 41 connected with the rack housing 10, a stator 42 and a rotator 43 which are installed in the interior of the motor housing 41, and a motor shaft 44 which is fixedly installed in an inner side of the rotor 43 and cooperates with the rotor 43.
A ball screw 50 is installed in an outer surface of the rack bar 11 and is formed of a screw shape groove of a semi-circular cross section for thereby converting a rotational force of the motor shaft 44 into a reciprocating force of the rack bar 11. A ball nut 60 which has a screw shape groove of a semi-circular cross section and a ball(not shown) is installed in the interior of the same, corresponding to a ball screw 50.
At this time, the ball nut 60 includes a return structure in such a manner that a ball circulates in the interior of the same. An end of the motor shaft 44 in which the ball nut 60 is installed is outwardly expanded so that the ball nut 60 is inserted thereinto, and a front end of the same is bent and fixes the inserted ball nut 60 in an axial direction and a radius direction.
In addition, A bush 13 is installed in the interior of an intermediate side of the rack housing 10 for thereby preventing a movement of the rack bar 11 in an axial direction and a drooping of the intermediate side of the same. Therefore, the rack bar 11 is supported by three points by a support yoke(not shown) of one side of the bush 13 of the intermediate side and the ball nut 60 of the other side.
In addition, the both ends of the rack housing 10 are formed in hollow, and a rack stopper 70 is formed in order for the rack bar 11 to be movable therein. The outer side of the rack stopper 70 are divided into two portions for thereby connecting the tie rod 30 and the rack bar 11, and an inner ball joint 31 is installed between the two portions.
At this time, the inner ball joint 31 has a diameter larger than the inner diameter of the rack stopper 70. Therefore, when a lock to lock operation in which the rack bar 11 is moved in the direction of one end in maximum is performed, the inner ball joint 31 is closest to the rack stopper 70, so that the movement is limited for thereby determining a stroke of the rack bar 11.
However, the above described conventional rack assist type electric power steering system has the following problems.
First, there is a problem in the supporting structure of a relatively thin and longitudinal rack bar 11.
The rack bar 11 reciprocates in the longitudinal direction in the interior of the rack housing 10. The first support point of the same is a portion in which a pinion 22 of a lower portion of the input shaft 21 is connected, and the above support point is a portion in which the support yoke which assembling in a back side of the rack housing is continuously pressed.
The second support point is a ball nut assembling portion which is assembled at an end portion of the other side of the rack bar. This portion is installed in an inner diameter side of the housing.
In the case that the first and second support points are supported, a drooping problem occurs in the intermediate portion. In order to prevent the above drooping problem at the intermediate portion, the bush 13 is interposed in the inner diameter portion of the rack housing.
However, in the case that the assembling is performed by inserting the bush 13 into an intermediate portion, the fabrication process and assembling process are complicated.
In the most preferred examples, there should be provided a first support point by the support yoke which supports a rack gear portion by positioning the ball nut at an intermediate portion of the rack bar, a second support point by the ball nut 60 of the intermediate portion, and a third support point by the bush 13 inserted at the end portion.
However, in order to position the ball nut at an intermediate portion of the rack bar based on the above described manner, since the section of the ball screw formed in the rack bar is more elongated rather than the section in which the ball nut actually runs, so that the section of the ball screw is unnecessarily extended, and the fabrication cost is increased.
In addition, the vertical load applied to the rack bar since the ball nut is positioned at an end portion of the rack bar is most applied at the ball nut among three points(support yoke, bush and ball nut), for thereby generating a vibration and noise at the ball nut, and the durability of the ball screw is decreased.
Second, the sensor of the connector and motor assembly is positioned at an intermediate portion of the steering system which includes the rack bar, the sensor is located nearest a exhaust manifold(not shown) of an engine which is located at a center portion of an engine room. Therefore, even when the function of a heat resisting property is maximized, the sensor is easily damaged by a exhaust heat of the engine, and the life span of the same is decreased.
Namely, as shown in FIG. 1, in the structure in which the ball nut is positioned at an end portion of the rack bar, the motor assembly is located at an intermediate portion, and the connector which is an accessory of the motor assembly positioned at an intermediate portion should be positioned at an intermediate portion, the system is largely affected by a exhaust heat which is generated in the exhaust manifold.
Third, the drooping problem of the rack bar occurs due to a self-weight which is vertically applied to the rack bar in the vehicle. A certain interference may occur between the ball screw of the rack bar and the rack stopper by the above drooping problem. At this time, an abnormal friction problem may occur due to the interference.
Fourth, in the conventional construction, since the rack housing, the motor housing connected to the rack housing and the motor assembly which includes the motor housing are assembled at the same time, the test(durability, anti-vibration, noise, performance, etc.) of the motor should be performed after the assembling process.
Therefore, in the case that the tests are not successful because the state of the motor is bad, the whole steering system may have an error.